The Hinder End of the Skull in Merycopo- Tamus and in Hippopotamus Minutus by Helga S
Total Page：16
File Type：pdf, Size：1020Kb
THE HINDER END OF THE SKULL IN MERYCOPO- TAMUS AND IN HIPPOPOTAMUS MINUTUS BY HELGA S. PEARSON, D.Sc. University College, London 1. MERYCOPOTAMUS IN 1839 Falconer1 first described Merycopotamus as a Hippopotamus. In 1876 Lydekker2, though placing it among the Anthracotheriidae, still regarded it as a link between that family and the Hippopotamidae; a year later3 he described its axis and astragalus as almost indistinguishable from those of the anthra- cothere "Hyopotamus" bovinus (Ancodus) from Bracklesham, but still main- tained that the skull and lower jaw revealed affinities with the Hippopotamidae. Recent workers place it without reservation among the Anthracotheriidae; Pilgrim and Forster Cooper have shown that in the Bugti beds of Baluchistan and other deposits of Oligocene or Lower Miocene age in India a rich variety of anthracotheres is to be found, and that while some have the typical five cusps on their molar teeth, and some have only four as in Merycopotamus, there are all stages intermediate between these in the degree of reduction of the protoconule. In figs. 1 and 2 of this paper I have attempted to reconstruct the glenoid, otic and occipital regions of the Merycopotamus skull, regions which are not clear in the old lithographs of the Fauna Antiqua Sivalensis4 and not much clearer in the new figure given by Lydekker in 18865. The present reconstructions are based on four skulls in the British Museum (Natural History); three of these, labelled M. dissimilis Falconer and Cautley, No. 18441, M. nanus Lydekker, Nos. 16551 and 16552, are those figured in the Fauna Antiqua Sivalensis, the fourth skull, No. 8583, is that of a young animal from Yenanyoung, Upper Burma. The structure of these regions of the skull is unquestionably like that of the anthracotheres6. The glenoid surface is of the same broad, flat type, smoothly curving upwards postero-laterally at the side of the post-glenoid 1 Palaeontological Memoir8, vol. I, pp. 130-8 et 8eq. 2 Rec. Geol. Surv. Ind. vol. nx, p. 153; see also correction in footnote Pal. Ind. ser. x, vol. iI, 1883, p. 165 (pt v, p. 24). 3 Rec. Geol. Surv. Ind. vol. x, p. 34. i Plate 67, figs. 1, 3, 5. 6 Pal. Ind. ser. x, vol. Iv, pt I, suppl. 1, plate VI, fig. 1. 6 See descriptions by present writer in Phil. Trans. ser. B, vol. ccxv, 1927, pp. 421-7 (Bra- chyodu Gorringei, B. borboniew, Ancodw velaunwB, etc.); also photograph of Brachyodus giganteu8 Pilgrim, Pat. I4. W.S. vol Iv, no. 2, 1912, plate XV, 238 Helga S. Pearson process. There is a similar compression of the parts behind the glenoid: the post-glenoid process leans backwards against the wide, plate-like post- tympanic process, the "neck" of the tympanic being squeezed into a flat crest between the two. The exoccipital is splayed out into another flat plate against the back of the post-tympanic process; above this plate, on the posterior surface of the squamosal, is a depressed area which is deeper on some skulls of this genus than on others. The tympanic bulla is similar in shape to that of all those anthracotheres in which I have seen it preserved uncrushed (e.g. Ancodus velaunus from Ronzon); its outline may not be quite correct in fig. 1, as it has there been reconstructed from several incomplete specimens, but it must be very nearly correct. The paroccipital process is not preserved intact in any of the skulls: most of it is present in No. 16552, but as it is embedded in the matrix only its postero-lateral border and broken tip can be seen. The skull differs from that of any anthracothere that I know in the re- latively high position of the glenoid surface which is distinctly dorsal to the level of the basis cranii. The glenoid has remained close to the bulla as in Brachyodus and not passed out laterally as in Ancodus velaunus, nor is the post- glenoid compression so extreme as in the latter form. The post-glenoid process is not so long and pendent as in some anthracotheres; owing to this, and to the higher position of the glenoid, the tympanic "neck" and post-tympanic process project well below the level of the post-glenoid.process instead of this process projecting ventrally to them as is usual among anthracotheres. These characters are probably in part primitive, though the raising of the glenoid surface above the basicranial level may have been a secondary tendency in Merycopotamus, as it appears to have been in the ruminants. There is no nearer approach to the construction of these parts in the Hippopotamidae than is found in all anthracotheres. 2. HIPPOPOTAMUS MINUTUS In 1902 Forsyth Major' pointed out the similarity between the pigmy hippopotamus remains collected by Miss D. M. A. Bate in Cyprus, and Cuvier's pigmy hippopotamus2 (Hippopotamus minutus Blainville, Hyopotamus minutus Kaup3). He maintained that in the shape of the molar teeth this species shows the most generalised condition of all the known hippopotami. Gervais4 had pointed out the similarity of Cuvier's pigmy hippopotamus to the West African pigmy hippopotamus Choeropsis liberiensis Leidy, and had accordingly named the former Choeropsis minutus, but Flower in 18875 contested the right of the Liberian form to generic distinction. P.Z.S. 1902, p. 107. 2 O88emen Fowsile, 1821, 2nd ed. vol. i, p. 322. 3 Cla.sification der Sdugethiere u. V6gel, Darmstadt, 1844, p. 78. 4 Zool. et-Pal. Gdn. 1867-9, prem. 96r. p. 250. 5 P.Z.S, 1887, P. 612. Skulls of Merycopotamus and Hippopotamus minutus 23g Fig. 1. Reconstruction of the basis cranii of Merycogpotamu, from three skulls in the British Museum. About three-quarters actual size. Except for section of broken tip, exact shape of paroccipital process (reconstructed with dotted line on right side of skull) is not shown by any of the skulls. I i. l Fig. 2. Reconstruction of the occiput of Merycopotamus, from three skulls in the British Museum. About three-quarters actual size.- 240 Helga S. Pearson In figs. 3 and 4 of this paper are given reconstructions of the Cyprian species, taken from two specimens collected by Miss Bate at a later date than the material on which Forsyth Major based his paper of 1902. They were used in modelling the skull of the restored skeleton which stands on exhibit in the fossil mammal gallery of the British Museum, a general account of which was given by Miss Bate in 19061. Each of these two specimens is the almost perfect hinder end of a skull, and except for the tympanic bullae, all four of which are broken ventrally, it has been an easy task to reconstruct the lacking parts of one with the aid of the other, or from the opposite side of the same specimen. /~~'~\ \'t Fig. 3. Reconstruction of the basis cranii of Hippopotamus minutes, from two skulls from Cyprus in the British Museum. About one-half natural size. These specimens show a very close agreement with the hinder end of the skull in the larger species of Hippopotamus living and extinct. The post- glenoid compression is extremely similar to that in the anthracotheres (see Pearson, loc. cit. pp. 433-5), but the paroccipital process lies in the same transverse plane as the post-tympanic squamosal, the splayed-out part of the exoccipital forming with the latter a single flattened plate which curves backwards at its outer, squamosal border, while its inner, exoccipital part projects down into the small paroccipital process. The occiput is much broader than in the anthracotheres, just as broad compared with the size of the skull as in the larger hippopotami, while owing to the post-tympanic part of the squamosal being pressed back, as described above, into the same transverse plane as the exoccipital, the occipital depression above the latter is very shallow and ill-defined. Furthermore, the glenoid surface of the Cyprian form is of the same laterally elongated type as in the larger hippopotami, narrow from behind forwards and sloping very gradually down on to the face of the post-glenoid process. This type of glenoid is readily distinguishable from that 1 Geol. Mag. N.S, Dec. v, vol. ii, 1906, p. 241. Skulls of Merycopotamus and Hippopotamus minutus 241 of the anthracotheres, though it might have been derived from one of similar pattern. Between the two fragments from Cyprus, the skull of a West African pigmy hippopotamus, and the semi-fossilised skull of a Hippopotamus mada- gascariensis which I have before me for comparison, there are various slight differences in the shape of the glenoid and the parts behind it. The Cyprian skulls agree, however, at least as well with the Madagascar as with the Liberian skull, and in no way appear to be more primitive or more nearly to approach the anthracotheres or the little Upper Eocene Cebochoerus (a possible relative of the hippopotamus stock). It seems, then, that the claim for the generic distinction of Hippopotamus minutus must rest on the characters of the teeth alone. Fig. 4. Reconstruction of the occiput of Hippopotamu8 minutus, from two skulls from Cyprus in the British Museum. About one-half natural size. SUMMARY The hinder region of the skull ofMerycopotamus shows only slight differences from that of all anthracotheres, and these differences are not in the direction of Hippopotamus. The hinder region of the skull of the Cyprian Hippopotamus minutes is closely similar to that of recent hippopotami and throws no further light on the affinities of the family. Anatomy LXII6 16.